Using generator expressions

[Jonathan Gossage]

Many thanks, all. It turned out that my problem was not fully understanding
the use and power of the unpack operator *. Using it to activate my
generator made things start to work. I changed the line where I invoked the
generator to:
    y = test1(*(a for a in st))

adding the unpack operator.

On Mon, Sep 25, 2023 at 11:15 AM Thomas Passin via Python-list <
python-list at python.org> wrote:

> On 9/25/2023 10:15 AM, Jonathan Gossage via Python-list wrote:
> > I am having a problem using generator expressions to supply the arguments
> > for a class instance initialization. The following example shows the
> > problem:
> >
> > class test1(object):
> >      def __init__(self, a, b):
> >
> >>          self.name = a
> >
> >          self.value = b
> > st = 'Programming Renaissance, Any'.split(', ')
> > y = test1(a for a in st)
> > print(f'Object values are: {y._a}, {y._b}')
> >
> > I would expect to get the values from the list generated by splitting the
> > string passed in as arguments to the new instance of test1, but instead
> > I get the generator expression by itself as a generator object. The
> > generator
> > expression is treated like a passive object instead of being run. If I
> had
> > wanted to pass the generator expression itself, I would have expected to
> > have
> > to use parentheses around the generator expression. Any suggestions on
> how
> > to
> > get the generator expression to run?
> > If I change the definition of the input arguments to *args I can capture
> the
> > arguments within __init__ but it is verbose and ugly. Also, I could
> accept
> > the
> > arguments from a Sequence and extract the Sequence members into the class
> > values. I would prefer my solution if I could get it to work.
> > Note that I tried generator expressions both inside parentheses and not,
> > without success.
> >
>
> You should get an error at the y assignment.  The argument of test1() is
> a generator, which would get assigned to the "a" argument, and there
> would be no "b" argument, which is an error.
>
> In any event, even if this were to work as you want, it would only work
> for strings that contain one comma.  And you ask for values like y._a,
> but y._a is never created, only y.a.  If you did convert the generator
> to a list, and if you fix the underscored variable names, it still
> wouldn't work because the arguments don't expect a list.
>
> Time to step back and figure out exactly what you actually want to do.
>
> --
> https://mail.python.org/mailman/listinfo/python-list
>


-- 
Jonathan Gossage